The presnet invention relates to electronic devices, and, more particularly, to thin film resistors for thin film devices and intergrated circuits.
Thin film devices and integrated circuits frequently require resistors as part of the circuitry, and thin film resistors, along with diffused and implanted resistors and polysilicon resistors, are typically the form of resistor used. Thin film resistors basically consist of a thin film of resistive material on a layer of insulating material with end contacts on the resistive material. See, generally, Glaser and Subak-Sharpe, Integrated Circuit Engineering, ch. 4 (Addison-Wesley, 1977).
Implanted and diffused resistors generally have a large temperature coefficient of resistance ("TCR") and cannot be trimmed like thin film resistors (such as by anodization which makes the film thinner or by laser removal of a portion of the film). Further, implanted resistors are apparently reproducible in silicon with sheet resistances up to 5,000 .OMEGA./.quadrature., but for gallium arsenide the material reproducibility and surface depletion layers provide additional limitations on implanted resistors. In contrast, thin film resistors with nichrome (20% chromium and 80% nickel) as the resistive material have a TCR in the order of 100 ppm/.degree.C. and sheet resistances in the range of 10 to 400 .OMEGA./.quadrature..
Thin film resistors can be fabricated in various geometric patterns, such as a meander, to increase resistance for a given area occupied, and the film thickness can be reduced to also increase resistance. However, as the device density increases and the design rules decrease for very large scale integration circuits, patterns are not feasible and the film thickness required for materials such as nichrome is too small to be practical. Thus there is a need for high resistivity materials with low TCR to use in thin film resistors.
Various materials with low TCR and high resistivity are available and include chromium silicide (24% chromium), chromium titanium (35% chromium), chromium silicon oxide (70% chromium), and stannic oxide; see K. Chopra and I. Kaur, Thin Film Device Applications (Plenum Press 1983). Of these materials, the cermet of chromium and silicon oxide is the most useful for large value resistors; however, this cermet is deposited by flash evaporation which leads to reproducibility problems and must be etched by ion milling. Sputtering deposition of this cermet is also possible, but the variability of target material leads to even greater reproducibilty problems than flash evaporation. Thus there is a need for high resistivity material with low TCR which can be deposited in a reproducible process and which can be easily etched and trimmed.